In Otto engines having direct injection, the fuel pressure is set as a function of the operating point of the Otto engine, up to a specified upper system pressure. In this context, the pressure range typically lies between 40 bar as the lower system pressure, and 200 bar as the upper one.
The fuel system of such an Otto engine may include a high-pressure system having a high-pressure reservoir (common rail). The generation of the required fuel pressure takes place, for example, by a high-pressure pump configured as a piston pump, which is driven mechanically by the Otto engine. The conveyed fuel quantity of the high-pressure pump is set by a quantity control valve that is set ahead of the high-pressure pump. The fuel pressure in the high-pressure system is measured by a high-pressure sensor and is supplied to a control unit. The control unit regulates the fuel pressure as a function of the operating point of the Otto engine by the corresponding actuation of the quantity control valve.
The fuel dosing into the individual cylinders of the Otto engine takes place by fuel injectors that are also actuated by the control unit.
A pressure limiting valve is provided, so that, in a fault case, for example, which causes a maximum delivery of the high-pressure pump, an admissible maximum system pressure is not exceeded. The pressure-limiting valve opens in response to a specified opening pressure, for instance, 250 bar, and enables a return flow of the fuel from the high-pressure system back into the low-pressure range of the fuel system before the high-pressure pump. The fuel pressure in the high-pressure system is thus limited to a maximum pressure corresponding to the opening pressure of the pressure-limiting valve.
The cold start of an Otto engine having direct injection may take place as a low-pressure start or as a high-pressure start. Thus, in European document EP 0 961 019 E1, a method is discussed for operating a direct-injecting internal combustion engine during the start, the internal combustion engine being able to be started optionally using a homogeneous or stratified operation, and for this, predetermined engine control parameters being calculated for the homogeneous and the stratified operation, as a function of the variable (TKW) characterizing the temperature of the internal combustion engine a selection being made between the operation using a homogeneous mixture and a stratified mixture, and in an operation using the stratified mixture, the injection being specified by the parameters injection quantity (MFF) and angle of injection (EOI). In the process, it is provided that the operation using the stratified mixture is only released when the fuel pressure (PRail) has reached a specified threshold value (P_THR) within a certain number of cycles of the internal combustion engine, and otherwise the operation using an homogeneous mixture is selected. The operation having the stratified mixture corresponds to a high-pressure start, while the operation having the homogeneous mixture represents a low-pressure start.
Advantages of the high-pressure start are the low quantity of fuel injected, reduced oil dilution as well as lower requirements on the configuration of the injector valves, since the static rate of flow Qstat is reduced, and the accuracy in dosing small quantities is increased thereby.
In a high-pressure start, the high pressure is built up as soon as the Otto engine begins to revolve, until a fixed pressure threshold is exceeded. Then there follows the injection and the ignition, and the engine starts. Typical pressure thresholds for the injection release lie between 20 bar and 80 bar, as a function of temperature. Before the main injection using ignition, a preinjection may be provided as high-pressure injection or low-pressure injection.
Otto engines are generally operated using fuel of hydrocarbons from fossil fuels based on refined petroleum, and to this fuel there is increasingly being added alcohol produced from renewable raw materials (plants), such as ethanol or methanol, in different mixture ratios. The ethanol proportion in fuel mixtures lies in a wider spread of 0% to 85%, depending on the market; in various countries, operation using pure alcohol is provided too. In the USA and Europe, a mixture is often used of 75-85% ethanol and 15-25% gasoline under the brand name E85. By taking on pure gasoline, all possible fuel compositions below an 85% ethanol proportion are thereby able to set in.
The cold start of an Otto engine having direct injection sets high requirements on the injection system in response to the use of a fuel mixture of gasoline and alcohol, even at temperatures between 10° C. and 20° C. Because of poor mixture preparation and the low inclination of alcohol to evaporate at low temperatures, the injection of a considerably larger quantity is required, compared to when the engine is hot. This increases the consumption and the emission of undesired exhaust gas components. At temperatures below 5° C., in the case of pure ethanol as the fuel, a reliable start is not possible without additional measures. Such additional measures may be heated injector valves or the additional injection of gasoline via a separate injection system, which means, however, a clear increase in costs in the manufacture of the injection system.